1. Field
This disclosure relates generally to micro electromechanical system (MEMS) devices, and more specifically, to self-poling piezoelectric MEMs devices.
2. Related Art
MEMS devices using piezoelectric thin films are attracting more and more attention, partly due to the fact that such devices enable low voltage operation and can easily convert electrical energy to mechanical energy or vice versa. One family of materials, namely perovskite type ceramics such as PZT, is particularly notable for its high piezoelectric constants. However, these piezoelectric materials have some critical draw backs: 1) the film of piezoelectric material needs to be electrically poled in order to have a usable piezoelectric property; and 2) the Currie Temperature of these piezoelectric materials is low. For example, PZT has a Currie Temperature (Tc) of around 360° C. At temperatures above one half of the PZT film's Currie Temperature (Tc/2), which corresponds to a approximate temperature of greater than 180° C. (>180° C.), the film starts to loose its piezoelectric property.
In most applications, finished MEMS based electronic devices need to go through a solder reflow process to be assembled onto a printed circuit board (PCB) to form a system. A most commonly used solder reflow temperature is around 260° C. Experiments have demonstrated that at a temperature of around 260° C., the piezoelectric coefficient of a PZT film decreases to less than ten percent (<10%) of its original post-poling value. Such a loss in piezoelectric coefficient disadvantageously degrades device performance and highly undesirable.
Accordingly, there is a need for an improved method and apparatus for overcoming the problems in the art as discussed above.